The present invention relates to the calibration of a display system employing a linear array of light modulators, and more particularly to a display system with a linear array of conformal GEMS devices and an image sensor for uniformity calibration.
Many display systems, including large screen TVs, conference room projectors, and digital cinema projectors generate an output image by employing a spatial light modulator that receives light from a light source to impart image information on a uniform light field. At present in such display systems, the light source is typically a white light lamp and the spatial light modulator is typically an area array containing liquid crystal devices or micromirror devices. Alternative projection display architectures, with one or more laser light sources and spatial light modulators that are linear arrays of electromechanical grating devices, have been demonstrated and show much promise for the future. For a display system with a linear array architecture, a scanning mirror is used to generate a two-dimensional scene from a projected one-dimensional image of the linear array.
A system and method for calibrating a display system with a linear array of Grating Light Valves (GLV) was published by R. W. Corrigan et al. in xe2x80x9cCalibration of a Scanned Linear Grating Light Valve Projection System,xe2x80x9d SID ""99 Digest, pp. 220-223. The calibration method described by Corrigan uses a single detector to calibrate an entire linear GLV array. The correction factors for each device on the array are obtained by focusing the output of the entire array on the detector, turning on the devices one by one, and measuring the resulting light output. In this device-by-device approach, the entire background signal of the array is measured in addition to the light output of a single device. If the background signal is sufficiently high, there are problems with the signal-to-noise ratio in determining the correction factors needed to produce a uniform flat field. This signal-to-noise ratio problem is especially pronounced in calibrating the flat field of a low intensity gray level. As an example, in the calibration of the 5% intensity flat-field for a GLV array with 1000 devices and a contrast ratio of 1000:1, the background signal is 20 times larger than the output intensity from a single device. An additional disadvantage is that a device-by-device approach does not readily produce accurate calibration factors for a uniform flat field because the outputs of adjacent devices may not add linearly (for example, the output intensity measured when two adjacent devices are turned on is not equal to twice the intensity of a single device). This output non-linearity is due to changes in the diffraction pattern that occur when adjacent devices are turned on. The diffraction effects are even more pronounced when using light sources with high coherence, such as lasers. A third disadvantage is that the device-by-device approach does not directly calibrate the flat field on the screen as seen by a viewer.
Calibration systems for displays with two-dimensional modulator arrays have been disclosed that sample the output and generate correction factors to improve image quality. Specifically, U.S. Pat. No. 5,386,253, entitled xe2x80x9cProjection Video Display Systems,xe2x80x9d by Fielding, Jan. 31, 1995, discloses the use of one or more point detectors that measure light reflected from the screen to determine the appropriate correction factor. The signal-to-noise ratio problem mentioned earlier becomes even more pronounced for this system. An alternative approach, described in U.S. Pat. No. 4,868,668, entitled xe2x80x9cSystem And Method For Image Adjustment In An Optical Projection System,xe2x80x9d by Tavernetti, Sep. 19, 1989, uses a two-dimensional CCD array to determine correction factors by sampling a portion of the light emerging from the two-dimensional modulator array. This particular approach cannot be used in a display system with a linear array modulator without the addition of data processing electronics to generate the appropriate correction factors for the linear array modulator from the two-dimensional CCD data.
A significant problem with display systems that employ linear array modulators is that even a slight non-uniformity in the projected one-dimensional image produces parallel bands in the two-dimensional scene that are noticeable and objectionable to many viewers. A calibration system is required to reduce these objectionable bands in the two-dimensional scene. There is a need, therefore, for an improved calibration system for a display with a scanned linear array modulator that corrects image banding and avoids the problems noted above.
The above need is met according to the present invention by providing a system and method for calibration of a display system with a linear modulator array that includes: a light source; a linear array of light modulating devices driven by a controller, a projection lens system that creates a line image of the linear array on a screen; a scanning mirror that sweeps the line image across the screen to form a two-dimensional scene; and an image sensor containing an area array of light sensitive detectors that captures image intensity data from the two-dimensional scene and transmits the image intensity data to the controller, wherein the controller converts the image intensity data into a one-dimensional array of correction factors for the linear array of light modulating devices.